The application range of liquid crystal display devices has been expanded to include applications such as aircrafts and automobiles, for example. Liquid crystal display devices are therefore required to have favorable display properties in various use environments. Here, important among the display properties are, for example, viewing angle characteristics and high speed response.
A widely used technique for achieving the wide viewing angle is the fringe field switching (FFS) mode. FIG. 10 illustrates schematic cross-sectional views of a conventional FFS mode liquid crystal display device in the states of (a) black display and (b) white display. The arrows illustrated in the state (b) in FIG. 10 indicate the direction of electric fields formed by voltage application. Hereinafter, the operation principle of the FFS mode is described with reference to FIG. 10. As illustrated in FIG. 10, an FFS mode liquid crystal display device has a basic structure in which a liquid crystal layer containing liquid crystal molecules 131 is provided between a pair of substrates 111 and 121, no electrode is formed on the substrate 111, and a thin-film transistor (TFT) 122, an insulating film 123, a common electrode 124, a dielectric layer 125, and a pixel electrode 126 are stacked in the given order on the substrate 121. The pixel electrode 126 is electrically connected to the TFT 122 via a contact hole 127.
In the FFS mode, the liquid crystal molecules 131 in the liquid crystal layer are aligned horizontally when no voltage is applied. Electric fields that are parallel to the TFT substrate (hereinafter, such electric fields are referred to as transverse electric fields) are generated by applying voltage between the common electrode 124 and the pixel electrode 126 which are provided to the TFT substrate. These electric fields rotate the alignment direction of each liquid crystal molecule 131. When no voltage is applied, the alignment direction of each liquid crystal molecule 131 is parallel or perpendicular to the axial directions of polarizing plates, so that black display is provided. When the liquid crystal molecules 131 are rotated by voltage application, the angle formed by the alignment direction of the liquid crystal molecules 131 and the axial direction of the polarizing plates is changed to produce gray or white display. The FFS mode providing display based on such an operation principle has an advantage of a wide viewing angle because this mode can control the display state while keeping the liquid crystal molecules 131 in a direction parallel to the substrates. When the state is switched from the voltage-on state to the voltage-off state, the alignment of the liquid crystal molecules 131 is not changed forcibly by the electric fields, but is returned to the original state as a result of reduction in the elastic energy. The FFS mode therefore has a disadvantage that the response speed becomes slow particularly at low temperatures where the viscosity of the liquid crystal is high.
The technique disclosed in Patent Literature 1 increases the response speed of liquid crystal display devices with transverse electric fields by disposing a pair of substrates that face each other and are each provided with a pair of electrodes, and applying voltage to the liquid crystal layer from both substrate sides.
Patent Literature 2, for example, discloses a technique of achieving high speed response which is called a super-fast response (SFR) mode. FIG. 11 illustrates schematic cross-sectional views of a conventional SFR mode liquid crystal display device in the states of (a) black display and (b) white display. The arrows illustrated in FIG. 11 indicate the directions of electric fields generated by voltage application. Hereinafter, the operation principle of the SFR mode is described with reference to FIG. 11. As illustrated in FIG. 11, an SFR mode liquid crystal display device has a basic structure in which a liquid crystal layer containing liquid crystal molecules 231 is provided between a pair of substrates 211 and 221, a first electrode 214 is stacked on the substrate 211, and TFTs 222, an insulating film 223, a second electrode 224, a dielectric layer 225, and a pair of drive electrodes 226 and 228 are stacked in the given order on the substrate 221. The drive electrodes 226 and 228 are electrically connected to the respective TFTs 222 via contact holes 227.
The SFR mode utilizes liquid crystal molecules having positive anisotropy of dielectric constant (Δ∈), and the liquid crystal molecules in the liquid crystal layer are initially vertically aligned by the vertical alignment film. Electric fields are always present between the counter substrate and the TFT substrate, and these electric fields perpendicular to the liquid crystal layer (hereinafter, such electric fields are referred to as vertical electric fields) also control the alignment of the liquid crystal molecules to a direction perpendicular to the counter substrate and the TFT substrate. In this state, black display is provided. When voltage is applied between the electrodes of the TFT substrate, electric fields in the horizontal direction are generated in the liquid crystal layer. The vertical electric fields and the transverse electric fields, when combined, form strong electric fields, and the liquid crystal molecules respond at a high speed to be parallel to the strong electric fields. In this SFR mode, electric fields are always present in all the cases of providing black display, gray display, and white display, and the display state is changed by changing the directions of the electric fields. As a result, high speed response is achieved regardless of the gray scale levels.